1. Field of the Invention
The present invention relates to a method of operating a fuel cell used in an electric vehicle or a stationary power generating system, for example.
2. Description of the Background Art
A fuel cell is a device produced by sandwiching a layer of an electrolyte between two electrodes (oxidant and fuel electrodes). The fuel cell converts chemical energy directly into electric energy by supplying a fuel to one of the electrodes and an oxidant to the other, thereby producing an electrochemical reaction between the fuel and the oxidant. There are several types of fuel cells. Today, solid polymer electrolyte fuel cells using a solid polymer electrolyte membrane having proton conductivity as the electrolyte layer are watched with keen interest as fuel cells suited for producing high output power.
Main constituent elements of a solid polymer fuel cell are an anode and a cathode which together constitute two facing electrodes, separators in which fuel gas or oxidant gas channels (grooves) are formed, and a polymer electrolyte membrane placed between the two electrodes. The anode, the polymer electrolyte membrane and the cathode are joined together to form a so-called membrane electrode assembly (MEA). Each electrode typically includes an electrode substrate (referred to also as a gas diffusion layer) made up of an electrically conductive porous material, such as carbon fiber, and a catalyst layer including a solid electrolyte component. The two electrodes and the polymer electrolyte membrane are joined into a single structure by hot pressing or a like method. A gas sealing layer is formed on an outer area of each electrode so that the gas supplied to an electrode surface would not leak to the exterior. Generally, the gas sealing layer is made of a heat-resistant plastic, such as polytetrafluoroethylene (PTFE).
A process of starting up the fuel cell is as follows. The oxidant, such as an oxygen-containing gas (e.g., air), is fed through an oxidant inlet formed in one of the separators and supplied to the oxidant electrode through the oxidant channels. On the other hand, a hydrogen-containing gas used as the fuel is fed through a fuel inlet formed in the other separator and supplied to the fuel electrode through the fuel channels. When the fuel cell is warmed up to a specific temperature and the oxidant and fuel electrodes are connected via an external circuit, a reaction expressed by chemical equation (1) below occurs on the oxidant electrode side and unreacted gas and water are discharged through the fluid channels and an air outlet:Positive electrode reaction: ½O2+2H++2e−→H2O   (1)whereas a reaction expressed by chemical equation (2) below occurs on the fuel electrode side and unreacted gas is discharged through the other fluid channels and a fuel outlet:Negative electrode reaction: H2→2H++2e−  (2)
Electrons produced by the aforementioned reactions flow from the fuel cell into the external circuit through the separator.
In a case where air (oxygen) is used as the oxidant, the oxidant (positive) electrode has a potential of 1.23 V (theoretical value) under no-load conditions while the potential of the oxidant electrode drops down to 0.7 to 0.8 V under normal operating conditions due to internal resistance of the fuel cell.
A phenomenon observed in the fuel cell thus structured is a decrease in catalytic activity occurring with the lapse of time when the fuel cell is operated at a relatively high electrode potential of 0.7 V or above. This phenomenon is considered to occur as oxides deposit on a surface of platinum (Pt) used in the oxidant electrode and an effective area of the active Pt surface decreases as a result of an increase in oxide-covered areas, thereby causing deterioration of fuel cell performance. It is therefore important to remove oxide deposits for maintaining normal fuel cell performance.
As an example, a non-patent document titled “A Study of Polymer Electrolyte Fuel Cell Performance at High Voltages. Dependence on Cathode Catalyst Layer Composition and on Voltage Conditioning” cowritten by Francisco A. Uribe and T. A. Zawodzinski, Electrochimica Acta, 47(2002) pp. 3799-3809 mentions that the authors studied a method of removing the oxides deposited on the Pt surface through a reduction process caused by lowering the potential of the oxidant electrode down to 0.2 V by producing a large impulsive current during fuel cell operation and found that this method worked effectively.
Another approach to the removal of oxide deposits on the Pt surface is a “cathode reconditioning process” described in Japanese Patent Application Publication No. 2003-115318, in which a fuel cell is operated to produce a current larger than produced under normal operating conditions such that the potential of the oxidant electrode drops down to 0.3 to 0.6 V at startup, during normal operation and stopping of the fuel cell.
Although the aforementioned approaches of the non-patent document and Japanese Patent Application Publication are effective for reconditioning the oxidant electrode by removing the oxides deposited thereon, it is necessary to flow a larger current than under normal operating conditions. To meet this requirement, wirings, an AC-DC converter, a circuit breaker and other components of a fuel cell system must have higher power ratings than would be necessary under normal operating conditions, resulting in an increase in an increased equipment size and a higher manufacturing cost.